The invention relates to an additive for production of positive active compositions for lead accumulators based on finely divided tetrabasic lead sulphate having a mean particle size of less than about 3 μm and finely divided silica, and to a process for producing this additive.
According to the prior art, the positive plates, in the course of production of lead accumulators, after the grid has been pasted with the positive active composition, are ripened and dried in batches or continuously in what are called ripening and drying chambers. The ripening forms tribasic (3PbOPbSO4) and/or tetrabasic (4PbOPbSO4) lead sulphates from the main ingredients in the form of lead oxide, water and lead sulphate. The plates are predominantly placed in stacks on pallets without dispersing them. Less commonly, they are placed on pallets without dispersing them or, in the specific case of double grids with lugs on the outside, hung loosely in frames.
For the ripening to give tribasic lead sulphate having crystal sizes of <10 μm, it is common practice to ripen the plates at about 55° C. over a period of 12 to 24 hours and then to dry them. According to the oxide used and the residual moisture content desired, the drying takes up to a few days.
Depending on the chemical and physical conditions, a phase transition from the formation of tribasic lead sulphate to the formation of tetrabasic lead sulphate takes place within the temperature range from 60 to 70° C. For the ripening to give tetrabasic lead sulphate, the plates, according to standard practice, are ripened at a temperature of typically >80° C. in water vapour for a few hours and then dried as in the case of tribasically ripened plates. A significant disadvantage in the case of such a ripening operation under water vapour is the formation of coarse tetrabasic lead sulphate crystals, with possible occurrence of crystal sizes >50 μm.
The ripened active composition of the positive plates is converted electrochemically to lead dioxide during the subsequent formation. The conversion of the basic lead sulphates becomes more demanding and laborious with increasing crystal size. The amount of electrical energy required for conversion of a coarsely crystalline structure is more than 25% higher than that for a structure comprising small crystals. A “structure comprising small crystals” here shall be understood to mean a material in which the crystal size is <10 μm. In the case of a structure comprising large crystals, crystals >30 μm are present. For complete formation, rest periods additionally have to be included. As a result of the introduction of the higher amount of energy and the necessity of including rest periods, the formation of coarsely crystalline tetrabasic lead sulphate generally takes much longer.
Ripening to give tetrabasic lead sulphates is advantageous in the case of lead accumulators with antimony-free alloys for the positive grids. Lead accumulators with antimony-free alloys for the positive grids and tetrabasically ripened, positive active compositions have a stable capacity under cycling and have a distinctly prolonged lifetime. Lead accumulators with antimony-containing alloys in the positive grids are additionally being replaced by antimony-free grids since these lead accumulators additionally have longer storability and visibly lower water consumption.
For this reason, there is a great interest in processes and means of ripening positive plates to give small crystals of tetrabasic lead sulphate. According to the prior art, two processes for this purpose should be emphasized:
According to a customary manufacturing practice, the plates are first tribasically ripened and advantageously dried below residual moisture content 0.5% by weight. This is followed by a water vapour treatment at temperatures of typically >80° C. for several hours. During this phase, the tribasic lead sulphate is converted to tetrabasic lead sulphate. The crystal size here remains virtually unchanged, provided that the moisture content during the water vapour treatment of the plates does not exceed about 2% by weight. In the case of excessively moist plates, growth to form coarse crystals of tetrabasic lead sulphate occurs. In the case of a properly executed process, after another subsequent drying operation, plates comprising small crystals of tetrabasic lead sulphate are present. A great disadvantage of this process lies in the long processing time. In addition, the paste-grid binding is poorer than in the case of the positive plates ripened directly to give coarse crystals of tetrabasic lead sulphate in water vapour. The size of the crystals of the tetrabasic lead sulphate cannot be controlled and is at crystal sizes of <3 μm. In the case of cyclic deep discharge of wet lead accumulators, this can lead to irreparable damage to the positive electrodes and hence to shortening of the service life of the lead accumulators.
In a second known process, tetrabasic lead sulphate which has previously been ground to a small size is added to the active positive composition during the production process. The ripening is effected in the same way as in the ripening discussed to give coarse crystals of tetrabasic lead sulphate under water vapour and preferably at temperatures above 80° C. The tetrabasic lead sulphate crystals added, which have been ground to a small size of diameter <1 μm act as nucleators and allow individualized plates to have controlled crystal growth to give a tetrabasic crystal structure comprising small crystals. This process is preferably performed continuously.
The disadvantage of this process is the need to disperse the plates, for example by suspending double plates at a distance or by means of a breathable barrier membrane between individual plates. It is now common practice in the production of plates of lead accumulators to stack these in stacks without spacers after pasting and to ripen them in the stack. The necessity of individualizing the plates thus constitutes considerable additional work. Thus, the current systems and techniques for plate production cannot be used without new additional equipment or considerable alterations. The dispersal of the plates by means of intermediate spaces or breathable barrier membranes leads to more space being required, as a result of which the capacity for plates in existing ripening and drying chambers is considerably reduced.
A further development of the above prior art is disclosed by WO 2004/059772 A2. This relates to an additive for production of positive active compositions for lead accumulators based on tetrabasic lead sulphate. This additive contains a tetrabasic lead sulphate having a mean particle size of less than about 3 μm and, for prevention of the agglomeration of the particles of the tetrabasic lead sulphate, finely divided silica. The finely divided silica is in hydrophobic and/or hydrophilic form and is especially fumed. This known additive exhibits extensive advantages, as shown in WO 2004/059772 A2. For instance, in the case of use of this additive, the entire spectrum of current plant technology and customary process operations is covered. Current pasting lines with downstream stacking systems for all standard ripening and drying chambers are usable without modification. In addition, this additive can be used to provide a continuous ripening and drying technique within an overall period of about 3 to 4 hours. In all applications addressed, the end product which occurs is small crystals of tetrabasic lead sulphates having crystal sizes of less than 10 μm. The plates obtained thereby are just as easy to form as those manufactured from tribasic lead sulphates.